This invention relates to tape drives, and, more particularly, to tape drives which employ DC motors, wherein the tape drive DC motors are arranged to longitudinally move a tape mounted on tape reels rotated by the DC motors under the control of a motor driver servo system.
Tape, such as magnetic tape, provides a means for physically storing data which may be archived or which may be stored in storage shelves of automated data storage libraries, and accessed when required. As an archival medium, tape often comprises the only copy of the data, such that the data must be written accurately and the tape must be handled with care to prevent damage.
The servo system which moves the tape longitudinally is typically very precise, and the servo system bases the longitudinal movement on the instantaneous velocity of the tape.
Tape drives frequently employ DC motors and a motor driver servo system for operating the DC motors, to produce well controlled motion parameters such as position, velocity, and tape tension. Precise control of tape velocity is required in order to correctly read and/or write data to the tape. For such control, a primary velocity signal is generated, for example, employing a formatted servo track on the tape to directly measure the tape velocity. In the IBM LTO Ultrium Magnetic Tape Drive based on LTO (Linear Tape Open) technology, the servo track is made up of a sequence of repeated flux transitions, which produce a signal in a servo read head. The signal is a repeated set of bursts, that is peak detected to produce digital signals that can be used by logic to measure the time spacing between the bursts. The logic provides a count value of a reference oscillator to represent the time spacing of the bursts. This count value varies inversely with velocity, and is used to compute the velocity of the tape as it passes over the read head.
The primary velocity signal is a very accurate, and direct, measurement of tape velocity. However, the primary velocity signal is not always available for use in controlling the tape drive. During acceleration, deceleration, and while moving the read head between servo bands, the primary velocity signal is not available. There may also be exceptional conditions, such a loss of the servo signal, which make the primary velocity unavailable. The tape drive must be capable of controlling velocity indefinitely without the aid of the primary velocity signal. During such times, an alternate, or secondary velocity signal is required.
One example of a secondary velocity signal is the back-EMF voltage in DC motors. As is known to those of skill in the art, the back-EMF voltage can be computed by subtracting the estimated winding resistance ohmic voltage from the motor voltage. The winding resistance ohmic voltage is computed by multiplying the estimated motor current by the estimated winding resistance. The angular velocity of a DC motor is calculated by dividing the back EMF voltage of the motor by the motor voltage constant of the motor. A servo system computes an estimated longitudinal velocity of the tape by multiplying the motor angular velocity by the radius of the tape at the reel the motor is driving.
Other examples of secondary velocity comprise encoders or analog tachometers. It may be possible to compute a secondary velocity from the Hall sensors, if they have a good enough resolution, as an example, a DC motor with 72 Hall states per revolution.
The secondary velocity is typically always available, but does not have the accuracy of the primary velocity signal. For example, in the case of the back-EMF measurement, the velocity signal includes error sources, such as motor voltage constant tolerances, winding resistance tolerances and thermal effects, motor driver signal tolerances, and motor current estimation tolerances, among others.
In accordance with the present invention, a calibration system, logic, and a method are provided for calibrating the longitudinal velocity of tape in a tape drive.
In one embodiment, logic is provided for operating a tape drive. The tape drive has a plurality of DC motors, each arranged to longitudinally move a tape mounted on tape reels rotated by the DC motors under the control of a motor driver servo system, the motor driver servo system computing velocities of each of the DC motors for given longitudinal tape velocities, and the tape drive has at least one rotation index sensor, such as Hall sensor, at each of the DC motors for sensing full revolutions of the DC motors. The logic:
operates each of the DC motors under separate control at steady state computed estimated velocity (Omega C) for at least one full revolution without a tape;
senses rotation index sensor(s) to indicate a full revolution of each of the DC motors;
measures the time of the full revolution of each of the DC motors;
determines the actual velocity (Omega A) of each of the DC motors by dividing the measured time of the full revolution of each of the DC motors into 2 times pi;
determines a calibration constant (K calib) for each of the DC motors by comparing the computed estimated velocity (Omega C) during the full revolution, to the determined actual velocity (Omega A); and
for the motor driver servo system, provides the calibration constants (K calib) for each of the DC motors for calibrating the motor velocity of each DC motor, whereby the motor driver servo system determines a longitudinal tape velocity for a tape mounted at the tape drive by averaging the tape velocity generated by each of the DC motors based on the calibrated motor velocity.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.